Cellular tendons for tlp

ABSTRACT

Tendon systems described herein may include a cellular tendon main body system having at least two essentially parallel pipe strings and may be used to moor a floating structure to a seabed. Hybrid tendon systems may include one or more tendon modules, where at least one tendon module includes a cellular tendon main body system. Where more than one module is used, the modules may be different types or designs of tendon systems, such as conventional single-string tendon or cellular tendon, according to the performance requirements of the module. Cellular tendon main body systems described herein enable the overall tendon, and subsequently the floating structure, to be used in ultra-deep waters and/or with heavy topsides.

FIELD OF THE DISCLOSURE

Embodiments disclosed herein relate to a tendon system for use withtension leg platforms (TLP). More specifically, embodiments disclosedherein relate to a tendon system that includes Cellular Tendonarrangement as the main body of at least one tendon or tendon module ora portion of one tendon or tendon module, designed and configured toenable use of the tendon, and thus use of TLPs, in ultra-deep watersand/or for heavy topsides, or to improve constructability andtransportation and installation and/or project cost for any water depth.

BACKGROUND

One type of offshore drilling and production platform is a tension legplatform, generally called a TLP, which utilizes tendons to support theplatform. The tendons have lower terminations that connect to pilings onthe sea floor. The upper ends connect to top connectors on the platform.The platform is de-ballasted after connection to the top connector,placing the tendons in tension.

One type of tendon includes a main body that is a single steel tubularformed of multiple segments connected together with mechanicalconnections. The pipes in the tendon segments have hollow interiors thatare sealed from sea water to provide buoyancy. Bulkheads may be locatedwithin the interior, dividing the hollow interior in separatecompartments sealed from each other. Use of conventional single steelpipe design for the tendon main body has technical and practicallimitations in meeting the combined stiffness and the tension-collapseresistance requirements, limiting the depth and topside payload at whichconventional tendon systems may be used.

U.S. Pat. No. 6,851,894 discloses tubular sections having threedifferent wall thicknesses. The upper section has a greater diameter butlesser wall thickness than an intermediate section, and the intermediatesection has a greater diameter but lesser wall thickness than the lowersection.

As TLP platforms are located in deeper waters, providing steel tubulartendons that can resist the hydrostatic pressure becomes an increasinglydifficult problem. For example, U.S. Pat. No. 7,422,394 discloses use ofa tendon having a varied diameter, decreasing in diameter with depth,and limiting the diameter to thickness ratio at depth to overcomecrushing force and hydrostatic pressure increases.

Various methods have been proposed to overcome this hindrance to use ofTLPs in extremely deep water. For example, another type of tendon is asolid cable, formed of composite fibers, such as carbon fibers (oftencalled a tether). Typically, a composite tendon, such as disclosed inU.S. Pat. No. 7,140,807, has an elastomeric jacket that encloses severalbundles of fibers. A spacer or filler fills the interior spacesurrounding the fibers. Steel terminations are located on the ends ofthe separate rods or sections of a composite tendon for connecting thesections to each other.

Composite fiber tendons are generally smaller in diameter than steeltubular tendons and weigh less. However, they are less buoyant, such astheir specific gravities being around 0.85 where 1.00 is consideredneutral. Having solid interiors, composite fiber tendons are able towithstand high hydrostatic pressures. However, the lack of buoyancylimits the usefulness of composite fiber tendons in very deep waterbecause a larger and more buoyant hull for the TLP is required in orderto maintain the required tension at the bottom connector. Also, fatigueof the upper portion of a composite fiber tendon can be a concernbecause of the high bending moments caused by TLP lateral motion. U.S.Pat. No. 4,990,030 discloses use of composite fibers in the interior ofa steel pipe section, which may provide the buoyancy, but as notedabove, the use of the single pipe would be unsuitable for deep watersdue to the high hydrostatic pressures.

SUMMARY OF THE DISCLOSURE

In one aspect, embodiments disclosed herein relate to a cellular tendonsystem that may be used to moor a floating structure to a seabed. Thetendon system may include a top tendon section configured to attach to afloating structure and a tendon bottom section configured to attach to afoundation. The tendon system may also include an upper transition unitconnected to the top tendon section, and a bottom transition unitconnected to the tendon bottom section. One or more tendon modules maybe disposed intermediate the upper and bottom transition units, at leastone tendon module including a tendon main body system comprising atleast two pipe strings connected proximate their upper ends to a firstmodule transition unit, which can be the same or different than theupper transition unit, and proximate their bottom ends to a secondmodule transition unit, which can be the same or different than thebottom transition unit. A tension leg platform may be moored to seabedusing such a tendon system.

In another aspect, embodiments disclosed herein relate to a cellulartendon system that may include a top tendon section configured to attachto a floating structure and a tendon bottom section configured to attachto a foundation. The tendon system may also include an upper transitionunit connected to the top tendon section, and a bottom transition unitconnected to the tendon bottom section. A tendon main body systemincluding at least two pipe strings is connected proximate their upperends to the upper transition unit and proximate their bottom ends to thebottom transition unit. The tendon system may be fully assembled onshoreand transported to an offshore location, where it may be submerged andconnected at its top end to a floating structure and at its bottom endto a foundation in the seabed. The floating structure may be, forexample, a tension leg platform.

In another aspect, embodiments disclosed herein relate to a method ofassembling a tendon system. The method may include assembling a tendonmain body section having two or more pipe strings. The two or more pipestrings may be connected at their top end and bottom ends to a tendontop segment and a tendon bottom segment, which may be, for example,configured to connect to a floating structure, a foundation, or anothertendon module.

In another aspect, embodiments disclosed herein relate to a hybridtendon system, including: an upper tendon module and a lowermost tendonmodule. The upper tendon module includes a top tendon section configuredto attach to a floating structure. The lowermost tendon module includesa bottom transition unit configured to attach to a foundation. At leastone of the tendon modules include a tendon main body system comprisingat least two pipe strings connected proximate their upper ends to anupper transition unit and proximate their bottom ends to a lowertransition unit. A tension leg platform may be moored to a seabed usingsuch a tendon system.

In another aspect, embodiments disclosed herein relate to a method ofmooring a floating structure. The method may include: transporting atendon system according to embodiments herein from an onshore assemblylocation to an offshore location; submerging the tendon system;connecting the tendon system to a foundation and connecting the tendonsystem to the floating structure.

Other aspects and advantages will be apparent from the followingdescription and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an elevation view of a tension leg platform anchored withtendons according to embodiments herein.

FIG. 2 is a side view of a tendon according to embodiments herein.

FIG. 3 is a top cross-sectional view of a tendon according toembodiments herein.

FIG. 4 is a detail view of a portion of the tendon illustrated in FIG.2.

FIG. 5 is an elevation view of a tension leg platform anchored withhybrid tendons according to embodiments herein.

FIG. 6 is a side view of a lower portion of a tendon according toembodiments herein.

FIG. 7 is a side view of a lower portion of a tendon according toembodiments herein.

FIGS. 8-10 are cross-sectional views of a tendon or portion thereofaccording to embodiments herein.

DETAILED DESCRIPTION

Embodiments disclosed herein relate to a tendon system for use with atension leg platform (TLP). More specifically, embodiments disclosedherein relate to a tendon system having at least one cellular tendonmain body portion designed and configured to enable use of the tendon,and thus use of TLPs, in ultra-deep waters and/or with heavy topsidepayload.

In a Tension Leg Platform (TLP), the tendons maintain the platformposition and control the TLP dynamic motions in various environmentalconditions to meet the operational requirements. An individual tendonconsists of three major parts: a tendon top segment (TTS), or topsection, for top interface at the platform, a tendon bottom segment(TBS), or bottom section, to connect to the tendon foundation at theseafloor, and a main body that links between the two. The main body maybe formed, according to embodiments herein, using one or more tendonmodules.

Tendons according to embodiments herein include one or more tendonmodules including a tendon main body section that has multiple metallicor composite tubular members or strings, bundled together, acting as themain body of the respective module of a tendon system for a Tension LegPlatform (TLP). These “cellular” tendons have unique features whichenable the use of TLP technology and application in hydrocarbonproduction in ultra-deep water and/or with heavy topside payload. Wherethe use of conventional tendons is technically and practically possible,the advantages of the cellular tendon over conventional tendons residein the technical robustness, and, depending on platform geographical andeconomical characteristics, local fabrication, and cost savings.

The cellular tendon system includes the tendon main body, and for asingle-module tendon system, an upper transition unit to interface withthe tendon top interface, and a bottom transition unit to interface withthe tendon bottom connection. Unlike the single carbon steel pipe usedas the main body in a conventional tendon design, a cellular tendonconsists of multiple metallic or composite tubular strings that arebundled together acting as one single body. Each individual stringconsists of multiple tubes or pipe segments connected to each other atthe ends of the tubes, such as by welding or mechanical connections. Thestrings may be arranged in parallel and assembled on-shore.

Centralizers or frames may be used to bundle the strings together.Buoyancy modules are used partially or fully along the cellular tendonlength to provide buoyancy necessary for the installation of thetendons. The TTS and TBS are assembled onshore and connected to thecellular tendon top transition unit and bottom transition unit,respectively. The cellular tendons, once fully assembled, may then betowed out to the site, upended, and installed.

A tendon system according to embodiments herein, such as a single-moduletendon system described further below with respect to FIGS. 1-4, mayinclude a top tendon section configured to attach to a floatingstructure, such as a TLP, and a tendon bottom section configured toattach to a foundation in the seabed. An upper transition unit may beconnected to the top tendon section, and a bottom transition unit may beconnected to the tendon bottom section. The tendon main body systemincludes at least two pipe strings connected proximate their upper endsto the upper transition unit and proximate their bottom ends to thebottom transition unit.

A tendon system according to embodiments herein, such as amultiple-module tendon system described further below with respect toFIGS. 5-7, may include a top tendon section configured to attach to afloating structure, such as a TLP, and a tendon bottom sectionconfigured to attach to a foundation in the seabed. The tendon main bodysystem includes two or more modules, including an upper tendon module, alowermost tendon module, and optionally one or more intermediate tendonmodules. At one of the tendon modules includes a tendon main body systemthat includes at least two pipe strings connected proximate their upperends to a first module transition unit and proximate their bottom endsto a second module transition unit.

The pipe strings are connected to the upper and bottom transition unitssuch that the pipe strings are mechanically coupled. The coupling(connection) may be made by welding or other means of mechanicalcoupling such as treated connections. For example, the pipe strings maybe welded to the upper and bottom transition units at the upper andlower ends of the individual pipe strings, respectively. Alternatively,the upper and lower ends of the pipe strings may be mechanically coupledto the upper and bottom transition units, respectively.

The pipe strings may be formed from steel, aluminum, or compositematerials. The inner and outer diameter of the pipe in the pipe stringsmay be constant throughout the length of the pipe string in someembodiments. In other embodiments, the inner diameter, the outerdiameter, or both, may vary along the length of the pipe string. Forexample, in some embodiments, the outer diameter of the pipes in thepipe string may remain constant, while the inner diameter decreases withdepth, the thickness of the pipe thereby increasing with depth. Othervariations in pipe diameter and thickness are also possible. The outerdiameter and wall thickness may be selected for each point along thelength of a tendon to carry tension from the buoyant and partiallysubmerged TLP (which consists of a nominal tension plus tensionvariations due to functional and environmental loads), to maintain anecessary tendon stiffness, to achieve a desired buoyancy, and towithstand the crushing forces of the surrounding sea.

The tendon main body system may include one or more centralizersdisposed along a length of the pipe strings. The centralizers may be,for example, metallic or non-metallic plates configured to space thepipe strings and arrange each the pipe strings in a parallelconfiguration (substantially parallel, in some embodiments, asmanufacturing process tolerances may provide for some minor deviationfrom parallel). The centralizers may also be connected to the pipestrings or otherwise configured such that the pipe strings aremechanically coupled. In this manner, the individual pipe strings form asingle operative unit. In some embodiments, the centralizers may includea non-ferric material associated with a metallic guide frame, describedfurther below.

To reduce consequences of flooding of the pipe strings, a plurality ofbulkheads may be mounted in the pipe strings. The bulkheads may formsealed compartments so that leakage at any point along the length of apipe section will flood only one compartment. The remaining sealedcompartments would maintain sufficient buoyancy to support the weight ofthe tendon. Bulkheads may be placed according to the choice of thedesigner. The bulkheads may be located at the end of a designated jointof pipe, or at selected intervals, for example. Bulkheads may be securedin a variety of manners, and in some embodiments are secured by weldingor mechanic locks.

The tendon main body system may also include one or more buoyancymodules disposed around at least a portion of the pipe strings. In someembodiments, buoyancy modules may be disposed around the individual pipestrings in the tendon main body; in other embodiments, the buoyancymodules may be disposed around the set of pipe strings forming thetendon main body. In some embodiments, the buoyancy modules may beformed from two or more sections disposed around respective portions ofthe pipe strings, and may be fastened to the pipe strings via non-ferricor metallic straps. For example, the buoyancy modules may be bundled tothe pipe strings using metal straps or straps made of high-strengthsynthetic fiber or fabric, with a mechanical lock connecting the ends.

The tendon main body system may also include one or more buoyancy arrestcollars connected to the pipe strings. The buoyancy arrest collars maybe provided to constrain the buoyancy modules to the cellular tendon. Insome embodiments, the buoyancy arrest collar includes one or moremechanic parts configured to engage corresponding profiles in thebuoyancy module.

The physical requirements of the tendons may vary with depth. Thus, insome embodiments, the tendon may be formed by two or more modules. Insome embodiments, for example, the tendon may include an upper tendonmodule and a lower tendon module; in other embodiments, the tendon mayinclude an upper tendon module, a lower tendon module, and one or moreintermediate tendon modules. As the design requirements (crush strength,etc.) of the shallower tendon modules may be lower than those used atdepth, a hybrid tendon according to embodiments herein may include, forexample, an upper tendon module, which may be a conventional tendon,such as a single-string tendon, and a lower tendon module or modules,which may be cellular tendons as described herein. In other embodiments,the tendon may include multiple cellular tendon modules. Where multiplecellular tendon modules are used along the length of the tendon, thedesign and physical requirements of the segments may be adjusted foroperating depth.

A hybrid tendon system according to embodiments herein, for example, mayinclude an upper tendon module and one or more lower tendon modules,including a lowermost tendon module. The upper tendon module may includea top tendon section configured to attach to a floating structure. Theupper tendon module may also include a lower tendon section configuredto attach to one of the lower tendon modules, such as to an upperportion of an intermediate tendon module or the lowermost tendon module.

The lower tendon modules may be configured to attach at their upper endsvia an upper transition unit to either (a) the lower tendon section ofthe upper tendon module or (b) a lower tendon section of an axiallyhigher lower tendon module. The lower tendon modules may also beconfigured to attach at their lower ends via a lower transition unit toeither (a) a foundation or (b) an upper tendon section of an axiallylower tendon module. The one or more lower tendon modules may be formedof a cellular tendon system, similar to that as described above, havinga tendon main body system including at least two pipe strings connectedproximate their upper ends to the upper transition unit and proximatetheir bottom ends to the lower transition unit.

The above described tendons may be used to secure any variety offloating platform to a seabed.

FIG. 1 is an elevation view of a floating platform 1, such as a TLP,moored to seabed 2 using tendon systems according to embodiments herein.Platform 1 may have a plurality of columns 4, and a horizontal section 6may extend between the columns 4. Columns 4 and horizontal pontoons 6,to provide buoyancy, and may be adapted to be selectively ballasted withwater. Platform 1 may also include one or more decks 8 for supporting avariety of equipment for offshore operations, such as drilling and/orproduction, such as through top tensioned risers 10.

Upper tendon supports 12 are mounted to platform 1 at each column 4 forattachment to an upper end 13 of each tendon 14. A minimum of twotendons 14 may be used to support at each tendon support 12, and thus aplatform 1 with four corners would have at least eight separate tendons14. The lower end 16 of each tendon 14 is secured to a piling orfoundation 18.

FIG. 2 illustrates a tendon 14 according to embodiments herein. Tendon14 includes a tendon top segment 20, tendon main body section 22, and atendon bottom segment 24. The top of the uppermost pipe segment intendon top segment 20 may be terminated with a connector assembly 25,commonly referred to as a length adjustment joint (LAJ) that is arrangedand designed to connect to the TLP hull.

The top of tendon main body section 22 is axially connected to thebottom of tendon top segment 20, and the bottom of tendon main bodysection 22 is axially connected to the top of tendon bottom segment 24.The bottom of the tendon bottom segment 24 is terminated with aconnector assembly 27, commonly referred to as a bottom latch assemblythat is arranged and designed to be received and locked into a piling 18or other foundation structure on the seabed.

Tendon main body section 22 includes multiple (i.e., two or more, smallquantities for large diameter seam pipes and large quantities for smalldiameter seamless pipes) metallic pipe strings 38. Each individual pipestring is formed from multiple pipe sections (segments, stands, orjoints) welded or mechanically coupled end-to-end. Alternatively, tendonmain body section 22 may include multiple composite pipe sections.

To facilitate connection of tendon main body section 22 to top andbottom segments 20, 24, an upper transition unit 26 and a bottomtransition unit 28 may be used. Transition units 26, 28 may includetapered sections 30, 32 expanding in diameter from the diameter of thetendon top segment and the tendon bottom segment, respectively,terminating at a large diameter section 34, 36. Large diameter sections34, 36 are sized appropriately for connection with the main body section22. The top and bottom of pipe strings 38 forming main body section 22may be welded or mechanically connected to transition units 26, 28,respectively.

For example, pipe strings 38 may be attached to the upper transitionunit 26 that interfaces with the tendon top segment 20. The pipes in thepipe strings 38 are welded to, or mechanically connected to, the uppertransition unit 26. A short taper 30 may or may not be used at the pipeup ends where the transition is made. At the top, the upper transitionunit 26 may include a weld profile to be welded at the bottom of thetendon top segment 20. The pipe strings 38 may also be attached to thebottom transition unit 28 that interfaces with the tendon bottom segment24. The pipes in the strings 38 are welded to, or mechanically connectedto, the bottom transition unit 28. A short taper 32 may or may not beused at the pipe bottom ends where the transition is made. At thebottom, the bottom transition unit 28 may have a weld profile to bewelded to the top of the tendon bottom segment 24.

The physical arrangement of pipe sections 38 may vary, and may depend onthe depth of service, the expected environmental conditions, and otherfactors. In some embodiments, the pipe strings 38 may be arranged suchthat the pipe strings run alongside each other. In other embodiments,the pipe strings 38 may be spaced apart from each other.

For example, in some embodiments, the pipe strings 38 may be spacedapart using a centralizer 40, a guide frame 42, or a combination of thetwo. Centralizers 40 may be made of non-ferric material, with or withoutthe metallic guide frames 42, and may be used to bundle the pipe stringstogether and keep all the strings acting as a mechanically compositeunit to meet the tendon strength and fatigue requirements. FIG. 2 andFIG. 3 show one possible arrangement of the centralizer 40.

Compartmentalization of the pipes may be used to reduce the in-waterweight of the tendon when a leak occurs in one pipe string 38. Forexample, as shown in FIG. 4, single or multiple compartments 44, 46 maybe formed inside the pipe strings, as well as the tendon top segment 20and the tendon bottom segment 24, by using bulkhead 48. The bulkheads 48may be used together with the guide frame 42 in some embodiments. Forexample, the guide frames 42 and bulkheads 48 may be inserted proximatean end of a pipe segment, facilitating connection to the next pipesegment forming the pipe string 38, spacing of the pipe strings, as wellas segregation of the pipe segments into sealed chambers 44, 46.

Buoyancy modules 50 may be used partially or fully along the CellularTendon length to provide buoyancy necessary for installation of thetendons. Material used to form the buoyancy modules may be syntheticfoams or other suitable light material. Open-bottom air cans or othertypes of buoyancy devices may also be used. Further, the buoyancy of thebuoyancy modules 50 may vary along the length of pipe strings 38.

The buoyancy modules 50 may also be used as a fabrication aid when thepipe strings 38 are assembled on shore. For example, using a centralizer40, internal to a pipe segment group, encapsulation of multiple pipesegments with a buoyancy module 50 may facilitate disposition of theends of the pipe segment for connection of guide frames 42, bulkheads44, and/or connection of the pipe segment to the subsequent pipesegment. In some embodiments, the buoyancy modules 50 may be fastened tothe pipe strings 38 by non-ferric or metallic straps 52, which may berecessed within a buoyancy module or may be placed at the surface arounda buoyancy module.

A buoyancy arrest collar 54 may be used, such as with the metallic guideframes 42 as shown in FIG. 4, to facilitate placement and retention ofthe buoyancy module 50 around the pipe strings 38, both duringconstruction and in service. For example, buoyancy arrest collar 54 mayinclude one or more mechanic parts configured to engage correspondingprofiles in the buoyancy module. In other embodiments, buoyancy arrestcollar may include a disc-like structure encompassing the pipe strings38, the outer portions of the disc engaging circumferential grooves inthe buoyancy modules 50, or engaging a terminal end of the buoyancymodules 50.

Referring now to FIG. 5, an elevation view of a floating platform 1,such as a TLP, moored to seabed 2 using hybrid or modular cellulartendon systems according to embodiments herein is illustrated, wherelike numerals represent like parts. As described above with respect toFIG. 1, the platform 1 may have a plurality of columns 4, and ahorizontal section 6 may extend between the columns 4. Columns 4 andhorizontal pontoons 6 provide buoyancy, and may be adapted to beselectively ballasted with water. Platform 1 may also include one ormore decks 8 for supporting a variety of equipment for offshoreoperations, such as drilling and/or production, such as through toptensioned risers 10.

Similar to the cellular tendon system described above, upper tendonsupports 12 are mounted to platform 1 at each column 4 for attachment toan upper end 13 of each tendon 14. Minimum two tendons 14 may besupported at each tendon support 12, and thus a platform 1 with fourcorners would have minimum eight separate tendons 14. The lower end 16of each tendon 14 is secured to a piling or foundation 18.

FIG. 5 illustrates two possible hybrid cellular tendons according toembodiments herein. Hybrid tendons 70 include an upper tendon module 72that is a conventional, single string, tendon connected to a lowertendon module 74 that is a cellular tendon, such as that described abovewith respect to FIG. 2. Hybrid tendons 76 include an upper tendon module78 that is a cellular tendon section, such as that described above withrespect to FIG. 2, connected to a lower tendon module 80, which may alsobe a cellular tendon similar to that as described above with respect toFIG. 2, where the modules 78, 80 may be of the same or different design,which may be varied to appropriately meet design criteria for intendeddepth of use.

FIG. 6 is a side view of a hybrid tendon 70 according to embodimentsherein, where like numerals represent like parts. Tendon 70 may includean upper tendon module 72 that is a conventional, single string, tendonconnected to a lower tendon module 74 that is a cellular tendon.

Upper tendon module 72 may include a tendon module top segment 81, atendon module main body section 82, and a tendon module bottom segment84. The top of the uppermost pipe segment in tendon top segment 81 maybe terminated with a connector assembly 25, commonly referred to as alength adjustment joint (LAJ) that is arranged and designed to connectto the TLP hull. Conventional tendon module 72 main body section 82 maybe formed from a single pipe string 86 including one or more segments 88of pipe axially connected end-to-end.

The top of tendon main body section 82 is axially connected to thebottom of tendon top segment 81, and the bottom of tendon main bodysection 82 is axially connected to the top of tendon bottom segment 84.The bottom of the tendon bottom segment 84 is terminated with aconnector assembly 87, which may be similar to a bottom latch assembly.

The connector 87 may be designed to be received and locked into areceiver assembly 90, having a receptacle 92, which may be designedsimilar to connector assemblies for attachment of a tendon to a piling,for example. The receiver assembly 90 may be connected, directly orindirectly, to bottom tendon module 74. For example, receiver assembly90 may be part of a transition module (not shown) for latching connectorassembly 87 and likewise latching to a connector assembly (not shown)forming an upper terminal end of bottom tendon module 74. In otherembodiments, such as illustrated in FIG. 6, connector assembly 87 mayform the upper terminal end 94 of bottom tendon module 74.

Bottom tendon module 74 may be similar to that as described above withrespect to FIGS. 2-4, where like numerals represent like parts. The topof bottom tendon main body section 22 is axially connected to the bottomof upper tendon module 72, as described above, and the bottom of tendonmain body section 22 is axially connected to the top of a tendon bottomsegment 24. The bottom of the tendon bottom segment 24 is terminatedwith a connector assembly 27, commonly referred to as a bottom latchassembly that is arranged and designed to be received and locked into apiling 18 or other foundation structure on the seabed.

FIG. 7 is a side view of a hybrid tendon 76 according to embodimentsherein, where like numerals represent like parts. Tendon 76 may includean upper tendon module 78 that is a cellular tendon connected to a lowertendon module 80 that is also a cellular tendon.

Upper tendon module 78 may include a tendon module top segment 101, atendon module main body section 102, and a tendon module bottom segment104. The top of the uppermost pipe segment in tendon top segment 101 maybe terminated with a connector assembly 25, commonly referred to as alength adjustment joint (LAJ) that is arranged and designed to connectto the TLP hull.

The top of tendon main body section 102 is axially connected to thebottom of tendon top segment 101, and the bottom of tendon main bodysection 102 is axially connected to the top of tendon bottom segment104. The bottom of the tendon bottom segment 104 is terminated with aconnector assembly 107, which may be similar to a bottom latch assembly.

The connector 107 may be designed to be received and locked into areceiver assembly 110, having a receptacle 112, which may be designedsimilar to connector assemblies for attachment of a tendon to a piling,for example. The receiver assembly 110 may be connected, directly orindirectly, to bottom tendon module 80. For example, receiver assembly112 may be part of a transition module (not shown) for latchingconnector assembly 107 and likewise latching to a connector assembly(not shown) forming an upper terminal end of bottom tendon module 80. Inother embodiments, such as illustrated in FIG. 7, connector assembly 107may form the upper terminal end 114 of bottom tendon module 80.

The top of bottom tendon main body section 122 is axially connected tothe bottom of upper tendon module 72, as described above, via connectorassembly 107, and the bottom of tendon main body section 122 is axiallyconnected to the top of a tendon bottom segment 124. The bottom of thetendon bottom segment 124 is terminated with a connector assembly 127,commonly referred to as a bottom latch assembly that is arranged anddesigned to be received and locked into a piling 18 or other foundationstructure on the seabed.

Upper tendon module 78 and bottom tendon module 80 may otherwise besimilar to the cellular tendons as shown and described above withrespect to FIGS. 2-4, where like numerals represent like parts.

In the various tendon modules 78, 80, as well as for intermediatemodules that may be placed between and connecting modules 78, 80 forembodiments including more than two tendon modules, the size, number,and physical arrangement of pipe sections 38 may vary, and may depend onthe depth of service, the expected environmental conditions, and otherfactors. In some embodiments, the pipe strings 38 may be arranged suchthat the pipe strings run alongside each other. In other embodiments,the pipe strings 38 may be spaced apart from each other. For example, asdescribed above with respect to FIGS. 2-4, in some embodiments, the pipestrings 38 may be spaced apart using a centralizer 40, a guide frame 42,or a combination of the two, as well as with a buoyancy module 50.

The upper tendon modules, such as modules 72, 78, as well as the bottomtendon modules, such as modules 74, 80, may include transition units130, similar to transition units 26, 28, connecting the respectivetendon main body segments 22, 82, 102, 122 to the tendon top segmentsand tendon bottom segments, as well as any intermediate connectorsegments used along the length of the hybrid tendon system.

Assembly of the cellular tendons according to embodiments herein may beperformed in any number of manners. The main body section 22 may beassembled pipe segment by pipe segment. In some embodiments, the top andbottom transition units and tendon top and bottom segments may beconnected in order of depth or height (top-to-bottom or bottom-to-top)along with the main tendon body pipe strings, or these components may beindividually assembled and connected to the main tendon body pipestrings following completion of the main tendon body pipe strings.

Methods of assembling a tendon system according to embodiments hereinmay thus include assembling a tendon main body section including two ormore pipe strings. The two or more pipe strings forming the tendon mainbody may be individually assembled by axially connecting two or morepipe segments to form a pipe string. The two or more pipe strings,having roughly equivalent length, may be axially connected at therespective ends to a tendon top segment and a tendon bottom segment. Anupper transition unit and a bottom transition unit may be used tofacilitate connection of the tendon main body section with the tendontop segment and the tendon bottom segment. As noted above, the tendonbottom segment and the tendon top segment may each be terminated with aconnector assembly.

As noted above, it may be desired to space the pipe strings forming thetendon main body section apart from one another. Guide frames,centralizers, and/or a buoyancy module may be used to achieve thedesired spacing, either during collective manufacture of the individualpipe strings or following manufacture of the individual pipe stringsindividually. A bulkhead may periodically be disposed in the pipestrings during assembly, and buoyancy arrest collars may be disposedalong the length of the pipe strings, either during collective assemblyof the individual pipe strings or following manufacture of theindividual pipe strings individually.

In some embodiments, the main tendon body pipe strings may be assembledcollectively as follows. A buoyancy module section, such as asemi-circular section, may be placed horizontally, with the interior ofthe module facing upward. The buoyancy module section may be configured,similar to that as illustrated in FIG. 3, having pockets 60 in which thepipe segments may be disposed or laid. Centralizer 40 may then bedisposed on the pipes and buoyancy modules, such as into a groove in thebuoyancy module section or by welding to pipes 38. The upper pipesegments may then be disposed on or connected to centralizer 40,subsequently encapsulated with a second buoyancy module section, andthen the buoyancy module sections may be cinched or secured to thestructure using straps 52. Disposing of a centralizer and buoyancymodule proximate each end of a pipe segment, such as within a couplefeet of an end, may thus allow for disposal of guide frames 42,bulkheads 48, and buoyancy arrest collars 54, as required, andconnection of two corresponding pipe segments forming an individual pipestring to be welded or otherwise connected to each other with relativeease.

One possible arrangement for buoyancy modules 50 is thus as illustratedand described with respect to FIG. 3, which may be used to facilitatetendon fabrication as described above. Other various arrangements forbuoyancy modules that may facilitate fabrication and spacing of strings38 during fabrication are illustrated in FIGS. 8-10.

As illustrated in FIG. 8, for example, a buoyancy module 150 may includetwo or more arms defining two or more recesses 154 in which the strings38 may be disposed. Non-ferric or metallic straps 52 may be placed atthe surface around the buoyancy module 150 and the strings 38. Asillustrated, buoyancy module 150 includes four arms 152/recesses 154;other embodiments may include three, five, six or more arms/recesses forplacement of the strings 38.

As illustrated in FIG. 9, as another example, a buoyancy module 160 maybe used to space strings 38 via corner recesses 162. Similarly, shapeshaving three, five, six or more corners may be used, where recessedcorners are designed for placement and/or retention of strings 38 duringfabrication and/or use of the cellular tendon or cellular tendonmodules.

As illustrated in FIG. 10, as another example, a buoyancy module 170 maybe used to space strings 38 via recesses 172. Recesses 172 may belocated proximate a corner of the shaped buoyancy module, as well asalong the length of the sides of the shaped buoyancy module. Asillustrated in FIG. 10, the buoyancy module 170 includes a recess 172proximate the top corner of the triangular buoyancy module 170, as wellas two recesses 172 located along the sides of the triangular buoyancymodule 170. In this manner, the bottom 174 of the buoyancy module may beused to support the strings 38 during fabrication, providing a flatsurface for disposal of the buoyancy module and strings 38 duringfabrication.

The buoyancy modules as illustrated in FIGS. 8-10 may provide for thedesired buoyancy of the cellular tendon modules. Additionally, these andsimilar configurations provide openings along the length of the string,allowing access to the pipes in the strings for inspection.Additionally, such embodiments may allow for removal and/or replacementof one or more strings offshore.

Similar to the embodiments for buoyancy modules illustrated in FIGS. 3and 8-10, guide frames may also be formed from a variety of shapes so asto properly space strings 38 and facilitate fabrication of the cellulartendons or tendon modules.

After assembly on shore, the cellular tendon system 14 as describedabove may be transported, or towed out, in single units, pairs, or otherconvenient connection numbers, to the offshore site. The towing can becarried out as surface tow or submerged tow. At the offshore site, thecellular tendon system 14 is upended and attached to the foundation 18.The tendon top segment 20 is pulled in and locked at the tension legplatform 1 tendon porch 12 via the length adjustment joint (LAJ) 25.After the tendon pretension reaches the design value, the tendon isready for the in-place services.

To add flexibility in the TLP construction schedule, the Cellular Tendonsystem 14 can be pre-installed. In the pre-installed condition, atemporary tendon supporting buoy may be used to maintain the nearvertical position of the tendons and meet the strength and fatiguerequirements from the wave and current loads, similar to those used inconventional tendon systems.

When multiple tendon modules are used to form a single tendon, such asillustrated in FIGS. 5-7, the cellular tendon modules may be fabricatedonshore as described above. After assembly onshore, the modules (such as72, 74 or 78, 80) may be towed out, in single units or in pairs, to theoffshore site. The towing can be carried out as surface tow or submergedtow. At the offshore site, the bottom tendon module may be upended andattached to the foundation. To add the flexibility in the TLPconstruction schedule, the bottom modules can be pre-installed prior toarrival of the upper modules. After towed to the site, the upper modulesmay be upended and latched, or otherwise attached, to the receivingmodule at the upper end of the lower module. The top tendon segment ofthe uppermost module is pulled in and locked at the TLP tendon porch viathe length adjustment joint (LAJ). After the tendon pretension hasreached the design value, the tendon is ready for the in-place services.In other embodiments, the upper and lower modules of the tendon systemmay be assembled offshore prior to submerging and upending.

As described above, tendons according to embodiments herein include atendon main body section that includes multiple metallic or compositetubular members, bundled together acting as the main body of a tendonsystem for a Tension Leg Platform (TLP). These “cellular” tendons haveunique features which enable the TLP technology and application inhydrocarbon production in ultra-deep water and/or with heavy topsides.In addition, the advantages of the cellular tendon over the conventionaltendons reside in the technical robustness, local fabrication, and costsavings.

The cellular tendon design has technical merits for actual fielddevelopments using TLPs with representative large, medium or smallpayloads in water depths between 1500 meters and 3000 meters, andpossibly deeper. The cellular tendon system has the following advantagesover the conventional tendons: provides sufficient vertical and lateralstiffness, while also ensuring sufficient resistance totension-collapse; provides more neutrally buoyant tendons that reduceTLP displacement and temporary buoyancy requirement during tendoninstallation; provides more structural redundancy for the tendon mainbody where the multiple tube construction in the Cellular Tendonwarrants additional structural redundancy in the event of one stringdamaged; enhances (and in some cases enables) the local fabricationcontent, where applicable; provides material cost savings as a result ofeliminating mechanical couplings; and provides installation cost savingsas a result of eliminating the need for heavy lift vessels.

While the disclosure includes a limited number of embodiments, thoseskilled in the art, having benefit of this disclosure, will appreciatethat other embodiments may be devised which do not depart from the scopeof the present disclosure. Accordingly, the scope should be limited onlyby the attached claims.

What is claimed:
 1. A tendon system, comprising: a top tendon sectionconfigured to attach to a floating structure; a tendon bottom sectionconfigured to attach to a foundation; an upper transition unit connectedto the top tendon section; a bottom transition unit connected to thetendon bottom section; and one or more tendon modules disposedintermediate the upper and bottom transition units, at least one tendonmodule including a tendon main body system comprising at least two pipestrings connected proximate their upper ends to a first moduletransition unit, which can be the same or different than the uppertransition unit, and proximate their bottom ends to a second moduletransition unit, which can be the same or different than the bottomtransition unit.
 2. The tendon system according to claim 1, comprisingone tendon module disposed intermediate the upper and bottom transitionunits, the tendon module including a tendon main body system comprisingat least two pipe strings connected proximate their upper ends to theupper transition unit and proximate their bottom ends to the bottomtransition unit.
 3. The tendon system according to claim 1, comprisingtwo or more tendon modules, including an upper tendon module, alowermost tendon module, and optionally one or more intermediate tendonmodules.
 4. The tendon system of claim 3, wherein the lowermost tendonmodule includes a tendon main body system comprising at least two pipestrings connected proximate their upper ends to a module transition unitand proximate their bottom ends to the bottom transition unit.
 5. Thetendon system of claim 3, further comprising one or more intermediatetendon modules, comprising: the upper tendon module including the toptendon section configured to attach to the floating structure and alower tendon section configured to attach to one of the intermediatetendon modules; the intermediate tendon modules: attached at their upperends via an upper transition unit to the lower tendon section of theupper tendon module; attached at their lower ends via a lower transitionunit to an upper tendon section of an axially lower tendon module; thelowermost tendon module: attached at its upper end to a lower tendonsection of an axially higher tendon module; attached at its lower endvia the bottom transition unit to the foundation.
 6. The tendon systemof claim 1, wherein the pipe strings are connected to the upper andbottom transition units such that the pipe strings are mechanicallycoupled.
 7. The tendon system of claim 1, wherein the pipe strings areformed from steel, composite, or aluminum.
 8. The tendon system of claim1, the tendon main body system further comprising one or morecentralizers disposed along a length of the pipe strings.
 9. The tendonsystem of claim 8, the centralizers configured to space the pipe stringsand mechanically couple the pipe strings.
 10. The tendon system of claim1, the tendon main body system further comprising one or more buoyancymodules disposed around at least a portion of the pipe strings.
 11. Thetendon system of claim 1, the tendon main body system further comprisinga buoyancy arrest collar connected to the pipe strings.
 12. The tendonsystem of claim 1, the tendon main body system further comprising one ormore guide frames disposed along a length of the pipe strings.
 13. Thetendon system of claim 1, wherein each of the pipe strings aresubstantially parallel to each other.
 14. The tendon system of claim 3,wherein one or more of the tendon modules comprises a single-string typetendon.
 15. The tendon system of claim 3, wherein the upper tendonmodule comprises a cellular tendon.
 16. A tension leg platform moored toa seabed using a tendon system as claimed in claim
 1. 17. A method ofmooring a floating structure, comprising: transporting a tendon systemaccording to claim 1 from an onshore assembly location to an offshorelocation; submerging the tendon system; connecting the tendon system toa foundation; and connecting the tendon system to the floatingstructure.
 18. The method of mooring a floating structure of claim 17,the tendon system comprising two or more tendon modules, including anupper tendon module, a lowermost tendon module, and optionally one ormore intermediate tendon modules, the method comprising: submerging thelowermost tendon module; connecting the lowermost tendon module to thefoundation; submerging an upper tendon module; connecting the uppertendon module to a lower tendon module; and connecting the upper tendonmodule to the floating structure.
 19. The method of mooring a floatingstructure of claim 17, wherein the tendon modules of the tendon systemare fully assembled onshore prior to transport.
 20. The method ofmooring a floating structure of claim 18, wherein the upper and lowermodules of the tendon system are connected offshore prior to submerging.21. A method of assembling a tendon system, comprising: assembling atendon main body section comprising two or more pipe strings; axiallyconnecting the top and bottom ends of the two or more pipe strings to atendon top segment and a tendon bottom segment, respectively.
 22. Themethod of claim 21, further comprising: disposing an upper transitionunit intermediate the top end of the two or more pipe strings and thetendon top segment; and disposing a bottom transition unit intermediatethe bottom end of the two or more pipe strings and the tendon bottomsegment.
 23. The method of claim 21, further comprising terminating thetendon bottom segment with a connector assembly; and terminating thetendon top segment with a connector assembly.
 24. The method of claim21, wherein assembling the tendon main body system comprises axiallyconnecting two or more pipe segments to form a pipe string.
 25. Themethod of claim 24, the assembling the tendon main body system furthercomprising spacing apart the two or more pipe segments or the two ormore pipe strings using at least one of a guide frame, a buoyancymodule, and a centralizer.
 26. The method of claim 21, furthercomprising disposing a bulkhead in at least one of the two or more pipestrings, the tendon top segment, and the tendon bottom segment.
 27. Themethod of claim 21, further comprising disposing a buoyancy arrestcollar in at least one of the two or more pipe strings, the tendon topsegment, and the tendon bottom segment.